Gastrointestinal: Unusual pathology of an anorectal lesion mimicking a benign laterally spreading tumor.

Abstract

A 70-year-old woman was referred to our institution for the diagnosis and treatment of an anorectal lesion detected on colonoscopy performed at a previous hospital for hematochezia. Colonoscopy revealed a 32-mm flat elevated lesion in the lower rectum with an anal component extending to the dentate line (Fig. 1a). Chromoendoscopy with indigo carmine spraying demonstrated a papillary surface (Fig. 1b). Magnifying endoscopy with narrowband imaging (NBI) displayed an irregular vessel resembling the intrapapillary capillary loop (IPCL) of the Japan Esophageal Society Classification Type B1 (Fig. 1c). Magnifying endoscopy with 0.05% crystal violet staining revealed the absence of discernible glandular ducts, hindering the evaluation of the pit pattern (Fig. 1d). The endoscopic appearance resembled a large adenoma or condyloma acuminatum; however, the absence of a pit pattern indicated that the lesion was covered with squamous epithelium and not columnar. Moreover, the irregular vessel pattern suggested a neoplastic lesion. Based on these observations, the lesion was diagnosed as a neoplasm originating from the squamous epithelium of the anal canal. Thus, we performed endoscopic submucosal dissection for complete elimination. Histological examination revealed a papillary growing squamous cell carcinoma (SCC) exhibiting low-grade and high-grade atypia and abundant dilated vessels (Fig. 2a). The tumor extended from the anal transitional zone (ATZ) to the rectum and entrapped the rectal crypts (Fig. 2b). Some tumor cells had a clear cytoplasm, indicative of koilocytosis (Fig. 2c), and positive p16 immunohistochemical staining suggested human papillomavirus (HPV) infection (Fig. 2d). The final diagnosis was HPV-associated SCC in situ originating from the ATZ with rectal extension. Anal canal cancer (ACC) is a rare disease that accounts for only 0.3% of all malignancies and is strongly associated with high-risk HPV (HPV-16 and HPV-18). This case was unusual because of the extensive infiltration of the columnar epithelium. Therefore, it might be misidentified as a rectal adenoma. Early ACC shows irregular vessel patterns reminiscent of esophageal SCC on magnified endoscopic NBI, which has been reported to be useful in diagnosing ACC. However, laterally spreading tumor (LST) can exhibit similar irregular capillary vessels. The difference in endoscopic appearance between ACC and LST can be explained by their different epithelial anatomies. ACC develops in the squamous cell epithelium and has no orifice or crypt. In contrast, LST originates from the columnar epithelium and has crypt openings. Condyloma acuminatum has a similar endoscopic appearance but with milder vascular dilatation and caliber changes. In conclusion, HPV-associated anal canal cancer can be suspected in papillary tumors with an irregular esophageal SCC-like vascular pattern and absence of glandular structures. Piecemeal endoscopic mucosal resection can be selected for large adenomas; conversely, en bloc resection is the preferred method for ACC to mitigate the risk of recurrence. To ensure the implementation of appropriate treatment, more attention should be paid to lesions extending along the anal canal to the rectum.